The book of Andrew Kydd is about the role of trust and mistrust in international relations and the Cold War. Its basic assumption is that when states can trust each other, they can live at peace, provided that they are security seekers, uninterested in expansion for its own sake.

Trust is a central issue in international relations (IR), and that centrality is exemplified in the most important struggle of the second half of the twentieth century, the Cold War. The book of Andrew Kydd is about the role of trust and mistrust in international relations and the Cold War. Its basic assumption is that when states can trust each other, they can live at peace, provided that they are security seekers, uninterested in expansion for its own sake. States that are security seekers therefore pay close attention to the motivations of others, attempting to determine who is a fellow security seeker and who is more inherently aggressive. Trust, in this book, is defined as a belief that the other side is trustworthy, that is, willing to reciprocate cooperation, and mistrust as a belief that the other side is untrustworthy, or prefers to exploit one's cooperation.

Two of the most important questions asked about the Cold War are why it began and why it ended when it did. Another key question is how European states managed to cooperate with each other and eventually with Germany so soon after a devastating war that sowed deep fears and hatreds. Kydd argues that these three questions are all related to international trust.

Explanations of the origins of the Cold War in terms of trust are not new in IR. The 'post-revisionist' school of Cold War historiagraphy, as well as 'traditionalist' and 'revisionist' accounts have all traced the origins of the Cold War to mistrust. The question of European cooperation and German rehabilitation is also a matter of trust. So is the question of the end of the Cold War. Thus, trust plays an important role in the debates about the beginning and end of the Cold War, and about European cooperation. The fact that many of these debates remain unresolved highlights the need for a better theoretical understanding of trust and cooperation in international relations. Towards this end, this book develops a theory of how trust affects cooperation between two actors as well as in larger groups, how it is eroded through aggressive behaviour, and how it is enhanced through cooperative gestures designed to reassure.

As the author highlights, there are four implications of his theory of trust: First, cooperation requires a certain degree of trust between states. The threshold of trust required for cooperation depends on a set of variables including a state's relative power and costs of conflict. Second, though conflict between trustworthy states is possible, when we see conflict it is a sign that one or both of the states are likely to be untrustworthy. Third, in multilateral settings, hegemony – the presence of a very powerful state – can promote cooperation, but only if the hegemon is relatively trustworthy. Fourth, if two parties are genuinely trustworthy, they will usually be able to reassure each other of this fact and eventually cooperate with each other. The key mechanism that makes reassurance possible is 'costly signalling', that is, making significant gestures that serve to prove that one is trustworthy.

With respect to the Cold War, these implications support three arguments. First, the Cold War was most likely a product of expansionist drives on the part of the Soviet Union, not a mutual desire for security accompanied by mistrust. Second, the European states were able to cooperate with each other, the United States, and Germany after World War II because the United States, as a trustworthy hegemon, enabled them to overcome serious mistrust problems. Finally, the Cold War was ended through a process of costly signalling. Gorbachev made a number of dramatic gestures that increased Western trust and dispelled the suspicions that underlay the forty-year conflict.

Two introductory chapters are followed by three pairs of chapters which constitute the main body of the book. Chapter 3 focuses on the spiral model, a key component of defensive realism, and analyses how trust can be eroded by competitive behaviour. Chapter 4 looks at the beginning of the Cold War and the increasing distrust between the United States and the Soviet Union as an instance of this phenomenon. The central argument here is that each side, but particularly the Soviet Union, harboured non-security-related motivations for expansion that would have led them to exploit, not reciprocate the cooperation of their rival.

The next pair of chapters examines trust and cooperation under hegemony. Chapter 5 presents a trust game involving multiple actors, which provides the tools to analyse how trust impacts upon cooperation when there are many actors with different interests, geographical situations, and relative capabilities. Chapter 6 applies the model to post-World War II European cooperation between the United States, Germany, and the rest of Europe. It shows that for hegemony to promote cooperation, the hegemon must be relatively trustworthy, in comparison to other states.

The final pair of chapters focuses on reassurance, looking at how states starting from a position of mistrust can reassure each other about their motivations and promote cooperation. Chapter 7 presents a model of reassurance via costly signals that shows how states can reassure each other by running risks of exploitation by the other side. Chapter 8 analyses the end of the Cold War in these terms, focusing on the process of reassurance that took place between East and West. The argument here is that the Soviet Union changed from an expansionist state to a security seeker, but that this change was not transparent. Therefore, Gorbachev implemented a policy of costly signalling to reassure the West.

In general, cooperative behaviour tends to reassure; cooperation has the effect of reassuring the other side and building trust. Indeed, reassurance can be defined as the process of building trust. It involves convincing the other side that you prefer to reciprocate cooperation, so that it is safe for them to cooperate. Mutual reassurance involves two parties, starting from a position of mistrust, simultaneously taking steps to reduce their mistrust. If trust can be increased then the two parties can eventually cooperate with each other, though they started out quite fearful of each other. To achieve reassurance it is necessary to go beyond simple assertions that you are trustworthy. The key mechanism that facilitates reassurance is costly signals. In the reassurance context, the signal must demonstrate that one is moderate, not out to get the other side, willing to live and let live, preferring to reciprocate cooperation.

Andrew Kydd argues that the decisive events that ended the Cold War can be interpreted as costly signals. Several events stand out in the period 1985-90 as particularly clear examples of signals by the Soviets that their motivations had changed. These include the 1987 Intermediate-range Nuclear Forces (INF) Treaty, the 1988 withdrawal from Afghanistan and announcement of conventional force reductions, and the 1989 revolutions in Eastern Europe. Another critical indication that Soviet motivations had changed was the process of democratisation beginning with the 19th Party Conference in 1988, and continuing on with elections to the Congress of People's Deputies in 1989 and the progressive transfer of power from the communist party to state institutions.

Although the Cold War now belongs to history, it still casts a long shadow over the present. It is remarkable that the author considers how the analysis of his book can be brought to bear on more current issues. During the 1990s there was a strong element of continuity in policy from the end of the Cold War. Some realists predicted that the institutional structures built up during the Cold War to link the United States to Europe and the European states to each other would collapse after the threat that led to their creation evaporated. Instead, these structures flourished. However, strengthening European institutions sometimes had an unfortunate side effect. US-Russian relations were harmed by the decision to expand the NATO alliance to include former Warsaw Pact members Poland, the Czech Republic and Hungary, and by the campaign against Serbia (a Russian ally) to secure autonomy for the Kosovo Albanians. The chief purpose of expanding NATO was to enlarge the zone of peace and mutual trust to Eastern Europe in hopes of preventing future wars there. The chief drawback was that it violated implicit pledges to Russia that NATO would not move closer to its borders, and, hence, fostered distrust on the part of a Russia that was left in the cold.

In the twenty-first century, US policy has shifted dramatically from this post-Cold War pattern as the administration of George W. Bush began pursuing a policy of unilateral neglect and then shifted to a policy of unilateral engagement. After the terrorist attacks of September 11, 2001, the US administration formally abandoned containment in favour of preventive war in the 2002 National Security Strategy. The United States would now attack potential threats before they could strike the United States, particularly states that were suspected of developing weapons of mass destruction or of having ties to terrorist groups. The invasion of Afghanistan and especially Iraq most clearly highlight the new trends in US foreign policy. In the case of Afghanistan, the Taliban's harbouring of Al Qaeda provided a rationale for war that was credible to other nations. The case of Iraq was much more controversial. Kydd argues that the United States, with its high minimum trust threshold, favoured preventive war while the Europeans and the rest of the world, with much lower minimum trust thresholds, favoured a continuation of the inspections regime and deterrence. The US and British invasion of Iraq without broader institutional backing has resulted in a serious weakening of international trust for the United States.

This decline in trust is one of the most serious problems the US faces in its efforts to protect itself against terrorism. The US therefore should address the roots of this mistrust. In order to counteract world suspicions about US foreign policy, Kydd thinks that US needs to implement a policy of reassurance by taking actions that a more expansionist US would reject.

This book makes extensive use of game theoretic models of international relations. These are abstract, stylised representations of certain aspects of the world, expressed in the language of mathematics. Such approach should not deter those who dislike formulae. Reading between the lines of Kydd's text is not likely to disappoint anyone.